Astronomical Curiosities Part 5

CHAPTER VII

The Minor Planets

Up to 1908 the number of minor planets (or asteroids) certainly known amounted to over 650.

From an examination of the distribution of the first 512 of these small bodies, Dr. P. Stroobant finds that a decided maximum in number occurs between the limits of distance of 255 and 285 (earth's mean distance from sun = 1), "199 of the asteroids considered revolving in this annulus." He finds that nearly all the asteroidal matter is concentrated near to the middle of the ring in the neighbourhood of the mean distance of 27, and the smallest asteroids are relatively less numerous in the richest zones.[115]

There are some "striking similarities" in the orbits of some of the asteroids. Thus, in the small planets Sophia (No. 251 in order of discovery) and Magdalena (No. 318) we have the mean distance of Sophia 310, and that of Magdalena 319 (earth's mean distance = 1). The eccentricities of the orbits are 009 and 007; and the inclinations of the orbits to the plane of the ecliptic 10 29' and 10 33'

respectively.[116] This similarity may be--and probably is--merely accidental, but it is none the less curious and interesting.

Some very interesting discoveries have recently been made among the minor planets. The orbit of Eros intersects the orbit of Mars; and the following have nearly the same mean distance from the sun as Jupiter:--

Achilles (1906 TG), No. 588, Patrocles (1906 XY), No. 617, Hector (1907 XM), No. 624,

and another (No. 659) has been recently found. Each of these small planets "moves approximately in a vertex of an equilateral triangle that it forms with Jupiter and the sun."[117] The minor planet known provisionally as HN is remarkable for the large eccentricity of its...o...b..t (038), and its small perihelion distance (16). When discovered it had a very high South Declination (61), showing that the inclination of the plane of its...o...b..t to the plane of the ecliptic is considerable.[118]

Dr. Bauschinger has made a study of the minor planets discovered up to the end of 1900. He finds that the ascending nodes of the orbits show a marked tendency to cl.u.s.ter near the ascending node of Jupiter's...o...b..t, a fact which agrees well with Prof. Newcomb's theoretical results. There seems to be a slight tendency for large inclinations and great eccentricities to go together; but there appears to be no connection between the eccentricity and the mean distance from the sun. The longitudes of the perihelia of these small planets "show a well-marked maximum near the longitude of _Jupiter's_ perihelion, and equally well-marked minimum near the longitude of his aphelion," which is again in good agreement with Newcomb's calculations.[119] Dr. Bauschinger's diameter for Eros is 20 miles. He finds that the whole group, including those remaining to be discovered, would probably form a sphere of about 830 miles in diameter.

The total ma.s.s of the minor planets has been frequently estimated, but generally much too high. Mr. B. M. Roszel of the John Hopkins University (U.S.A.) has made a calculation of the probable ma.s.s from the known diameter of Vesta (319 miles, Pickering), and finds the volume of the first 216 asteroids discovered. From this calculation it appears that it would take 310 asteroids of the 6th magnitude, or 1200 of the 7th to equal the moon in volume. Mr. Roszel concludes that the probable ma.s.s of the whole asteroidal belt is between 1/50th and 1/100th of that of the moon.[120] Subsequently Mr. Roszel extended his study to the ma.s.s of 311 asteroids,[121] and found a combined ma.s.s of about 1/40th of the moon's ma.s.s.

Dr. Palisa finds that the recently discovered minor planet (1905 QY) varies in light to a considerable extent.[122] This planet was discovered by Dr. Max Wolf on August 23, 1905; but it was subsequently found that it is identical with one previously known, (167) Urda.[123] The light variation is said to be from the 11th to the 13th magnitude.[124]

Variation in some of the other minor planets has also been suspected.

Prof. Wendell found a variation of about half a magnitude in the planet Eunomia (No. 15). He also found that Iris (No. 7) varies about a quarter of a magnitude in a period of about 6{h} 12{m}.[125] But these variations are small, and perhaps doubtful. The variability of Eros is well known.

The planet Eros is a very interesting one. The perihelion portion of its...o...b..t lies between the orbits of Mars and the earth, and the aphelion part is outside the orbit of Mars. Owing to the great variation in its distance from the earth the brightness of Eros varies from the 6th to the 12th magnitude. That is, when brightest, it is 250 times brighter than when it is faintest.[126] This variation of light, is of course, merely due to the variation of distance; but some actual variation in the brightness of the planet has been observed.

It has been shown by Oeltzen and Valz that Cacciatore's supposed distant comet, mentioned by Admiral Smyth in his _Bedford Catalogue_, must have been a minor planet.[127]

Dr. Max Wolf discovered 36 new minor planets by photography in the years 1892-95. Up to the latter year he had never seen one of these through a telescope! His words are, "Ich selsbt habe noch nie einen meinen kleinen Planeten am Himmel gesehen."[128]

These small bodies have now become so numerous that it is a matter of much difficulty to follow them. At the meeting of the Royal Astronomical Society on January 8, 1909, Mr. G. F. Chambers made the following facetious remarks--

"I would like to make a suggestion that has been in my mind for several years past--that it should be made an offence punishable by fine or imprisonment to discover any more minor planets. They seem to be an intolerable nuisance, and are a great burden upon the literary gentlemen who have to keep pace with them and record them. I have never seen, during the last few years at any rate, any good come from them, or likely to come, and I should like to see the supply stopped, and the energies of the German gentlemen who find so many turned into more promising channels."

Among the minor planets numbered 1 to 500, about 40 "have not been seen since the year of their discovery, and must be regarded as lost."[129]

CHAPTER VIII

Jupiter

This brilliant planet--only inferior to Venus in brightness--was often seen by Bond (Jun.) with the naked eye in "high and clear sunshine"; also by Denning, who has very keen eyesight. Its brightness on such occasions is so great, that--like Venus--it casts a distinct shadow in a dark room.[130]

The great "red spot" on Jupiter seems to have been originally discovered by Robert Hooke on May 9, 1664, with a telescope of 2 inches aperture and 12 feet focus. It seems to have existed ever since; at least the evidence is, according to Denning, in favour of the ident.i.ty of Hooke's spot with the red spot visible in recent years. The spot was also observed by Ca.s.sini in the years 1665-72, and is sometimes called "Ca.s.sini's spot."

But the real discoverer was Hooke.[131]

The orbit of Jupiter is so far outside the earth's...o...b..t that there can be little visible in the way of "phase"--as in the case of Mars, where the "gibbous" phase is sometimes very perceptible. Some books on astronomy state that Jupiter shows no phase. But this is incorrect. A distinct, although very slight, gibbous appearance is visible when the planet is near quadrature. Webb thought it more conspicuous in twilight than in a dark sky. With large telescopes, Jupiter's satellites II. and III. have been seen--in consequence of Jupiter's phase--to emerge from occultation "at a sensible distance from the limb."[132]

According to M. E. Rogovsky, the high "albedo of Jupiter, the appearance of the clear (red) and dark spots on its surface and their continual variation, the different velocity of rotation of the equatorial and other zones of its surface, and particularly its small density (133, water as unity), all these facts afford irrefragable proofs of the high temperature of this planet. The dense and opaque atmosphere hides its glowing surface from our view, and we see therefore only the external surface of its clouds. The objective existence of this atmosphere is proved by the bands and lines of absorption in its spectrum. The interesting photograph obtained by Draper, September 27, 1879, in which the blue and green parts are more brilliant for the equatorial zone than for the adjacent parts of the surface, appears to show that _Jupiter_ emits its proper light. It is possible that the constant red spot noticed on its surface by several observers, as Gledhill, Lord Rosse, and Copeland (1873), Russel and Bredikhin (1876), is the summit of a high glowing mountain. G. W. Hough considers Jupiter to be gaseous, and A. Ritter inferred from his formulae that in this case the temperature at the centre would be 600,000 C."[133]

The four brighter satellites of Jupiter are usually known by numbers I., II., III., and IV.; I. being the nearest to the planet, and IV. the farthest. III. is usually the brightest, and IV. the faintest, but exceptions to this rule have been noticed.

With reference to the recently discovered sixth and seventh satellites of Jupiter, Prof. Perrine has suggested that the large inclination of their orbits to the plane of the planet's equator seems to indicate that neither of these bodies was originally a member of Jupiter's family, but has been "captured by the planet." This seems possible as the orbits of some of the minor planets lie near the orbit of Jupiter (see "Minor Planets"). A similar suggestion has been made by Prof. del Marmol.[134]

Many curious observations have been recorded with reference to Jupiter's satellites; some very difficult of explanation. In 1711 Bianchini saw satellite IV. so faint for more than an hour that it was hardly visible! A similar observation was made by La.s.sell with a more powerful telescope on June 13, 1849. Key, T. T. Smyth, and Denning have also recorded unusual faintness.[135] A very remarkable phenomenon was seen by Admiral Smyth, Maclear, and Pearson on June 26, 1828. Satellite II., "having fairly entered on Jupiter, was found 12 or 13 minutes afterwards _outside the limb_, where it remained visible for at least 4 minutes, and then suddenly vanished." As Webb says, "Explanation is here set at defiance; demonstrably neither in the atmosphere of the earth, nor Jupiter, where and what could have been the cause? At present we can get no answer."[136]

When Jupiter is in opposition to the sun--that is, on the meridian at midnight--satellite I. has been seen projected on its own shadow, the shadow appearing as a dark ring round the satellite.

On January 28, 1848, at Cambridge (U.S.A.) satellite III. was seen in transit lying between the shadows of I. and II. and so black that it could not be distinguished from the shadows, "except by the place it occupied."

This seems to suggest inherent light in the planet's surface, as the satellite was at the time illuminated by full sunshine; its apparent blackness being due to the effect of contrast. Ca.s.sini on one occasion failed to find the shadow of satellite I. when it should have been on the planet's disc,[137] an observation which again points to the glowing light of Jupiter's surface. Sadler and Trouvelot saw the shadow of satellite I.

double! an observation difficult to explain--but the same phenomenon was again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of Detroit, Michigan (U.S.A.). He says that the satellite "was in transit nearing egress, and it appeared as a white disc against the dark southern equatorial belt; following it was the usual shadow, and at an equal distance from this was a second shadow, smaller and not so dark as the true one, and surrounded by a faint penumbra."[138]

A dark transit of satellite III. was again seen on the evening of December 19, 1891, by two observers in America. One observer noted that the satellite, when on the disc of the planet, was intensely black. To the other observer (Willis L. Barnes) it appeared as an ill-defined _dark_ image.[139] A similar observation was made on October 9 of the same year by Messrs. Gale and Innes.[140]

A "black transit" of satellite IV. was seen by several observers in 1873, and by Prof. Barnard on May 4, 1886. The same phenomenon was observed on October 30, 1903, in America, by Miss Anne S. Young and Willis S. Barnes.

Miss Young says--

"The ingress of the satellite took place at 8{h} 50{m} (E. standard time) when it became invisible upon the background of the planet. An hour later it was plainly visible as a dark round spot upon the planet. It was decidedly darker than the equatorial belt."[141]

The rather rare phenomenon of an occultation of one of Jupiter's satellites by another was observed by Mr. Apple, director of the Daniel Scholl Observatory, Franklin and Marshall College, Lancaster, Pa.

(U.S.A.), on the evening of March 16, 1908. The satellites in question were I. and II., and they were so close that they could not be separated with the 115-inch telescope of the Observatory.[142] One of the present writer's first observations with a telescope is dated May 17, 1873, and is as follows: "Observed one of Jupiter's satellites occulted (or very nearly so) by another. Appeared as one with power 133" (on 3-inch refractor in the Punjab). These satellites were probably I. and II.

Jupiter has been seen on several occasions apparently without his satellites; some being behind the disc, some eclipsed in his shadow, and some in transit across the disc. This phenomenon was seen by Galileo, March 15, 1611; by Molyneux, on November 12, 1681; by Sir William Herschel, May 23, 1802; by Wallis, April 15, 1826; by Greisbach, September 27, 1843; and by several observers on four occasions in the years 1867-1895.[143] The phenomenon again occurred on October 3, 1907, No. 1 being eclipsed and occulted, No. 2 in transit, No. 3 eclipsed, and No. 4 occulted.[144] It was not, however, visible in Europe, but could have been seen in Asia and Oceania.[144] The phenomenon will occur again on October 22, 1913.[145]

On the night of September 19, 1903, a star of magnitude 6 was occulted by the disc of Jupiter. This curious and rare phenomenon was photographed by M. Lucien Rudaux at the Observatory of Donville, France.[146] The star was Lalande 45698 (= BAC 8129).[147]

Prof. Barnard, using telescopes with apertures from 5 inches up to 36 inches (Lick), has failed to see a satellite through the planet's limb (an observation which has been claimed by other astronomers). He says, "To my mind this has been due to either poor seeing, a poor telescope, or an excited observer."[148] He adds--

"I think it is high time that the astronomers reject the idea that the satellites of Jupiter can be seen through his limb at occultation.

When the seeing is bad there is a spurious limb to Jupiter that well might give the appearance of transparency at the occultation of a satellite. But under first-cla.s.s conditions the limb of Jupiter is perfectly opaque. It is quibbling and begging the question altogether to say the phenomenon of transparency may be a rare one and so have escaped my observations. Has any one said that the moon was transparent when a star has been seen projected on it when it ought to have been behind it?"

Prof. Barnard and Mr. Dougla.s.s have seen white polar caps on the third and fourth satellites of Jupiter. The former says they are "exactly like those on Mars." "Both caps of the fourth satellite have been clearly distinguished, that at the north being sometimes exceptionally large, covering a surface equal to one-quarter or one-third of the diameter of the satellite."[149] This was confirmed on November 23, 1906, when Signor J. Comas Sola observed a brilliant white spot surrounded by a dark marking in the north polar region of the third satellite. There were other dark markings visible, and the satellite presented the appearance of a miniature of Mars.[150]

An eighth satellite of Jupiter has recently been discovered by Mr. Melotte at the Greenwich Observatory by means of photography. It moves in a retrograde direction round Jupiter in an orbit inclined about 30 to that of the planet. The period of revolution is about two years. The orbit is very eccentric, the eccentricity being about one-third, or greater than that of any other satellite of the solar system. When nearest to Jupiter it is about 9 millions of miles from the planet, and when farthest about 20 millions.[151] It has been suggested by Mr. George Forbes that this satellite may possibly be identical with the lost comet of Lexell which at its return in the year 1779 became entangled in Jupiter's system, and has not been seen since. If this be the case, we should have the curious phenomenon of a comet revolving round a planet!

According to Humboldt the four bright satellites of Jupiter were seen almost simultaneously and quite independently by Simon Marius at Ausbach on December 29, 1609, and by Galileo at Padua on January 7, 1610.[152] The actual priority, therefore, seems to rest with Simon Marius, but the publication of the discovery was first made by Galileo in his _Nuncius Siderius_ (1610).[153] Grant, however, in his _History of Physical Astronomy_, calls Simon Marius an "impudent pretender"! (p. 79).

M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890, twenty minutes before sunset.[154]

Humboldt states that he saw Jupiter with the naked eye when the sun was from 18 to 20 above the horizon.[155] This was in the plains of South America near the sea-level.

CHAPTER IX